Clinical and necropsy findings are described in 12 patients who had fatal aortic dissection with the entrance tear in the transverse aorta. The 12 patients represent 7% of the 181 cases of spontaneous aortic dissection seen at necropsy in the Pathology Branch, National Heart, Lung, and Blood Institute, during a 30-year period. The ages of the 12 patients at death ranged from 37 to 87 years (mean 67); 8 were men; 8 had a history of systemic hypertension, and 10 had hearts of increased weight. Diagnosis of aortic dissection was made during life in only 4 of the 12 patients. All 12 patients died of rupture of the false channel within 2 weeks of onset of signs or symptoms compatible with aortic dissection. The direction of aortic dissection from the entrance tear was entirely retrograde in 4 patients, entirely antegrade in 4 patients, and in both directions in 4 patients. Hemopericardium occurred in the first group, left hemothorax in the second group, and either in the last group. Of the 8 patients in whom the ascending aorta was involved, the retrograde dissection in each extended into the wall of aorta behind the sinuses of Valsalva, 6 had pulmonary adventitial hemorrhage, and 4 had involvement of the arch arteries by dissection. In the 4 patients who had strictly antegrade dissection, none had dissection involving the arch arteries. Thus, tear in the transverse aorta causes a dissection which is usually fatal, which often dissects retrogradely and which may mimic dissection from a tear in ascending aorta. Aortic dissection from a tear in transverse aorta requires immediate operative intervention.